Generally, it is important for a semiconductor element such as a semiconductor laser element and a high-frequency element used in the optical communication field and the like to efficiently release heat generated from the element during operation, in order to prevent malfunction or failure. Recently, technological development of semiconductor elements has promoted the speed, power, and high integration of the elements, and a demand for the heat dissipation has become increasingly more challenging. Thus, in general, a high thermal conductivity has been also required for heat dissipating components such as a heat sink, and copper (Cu) having a high thermal conductivity of 390 W/mK has been used.
On the other hand, the size of individual semiconductor element has been increased with an increase in output power, and problems of a thermal expansion mismatch between the semiconductor element and the heat sink used for heat dissipation have become apparent. To solve these problems, there is a need for development of a heat sink material exhibiting high thermal conductivity and a coefficient of thermal expansion equivalent to that of the semiconductor element. As such a material, a prior art proposes a composite of a metal and a ceramic, for example, a composite of aluminum (Al) and silicon carbide (SiC) (Patent Document 1).
However, the Al—SiC composite only has a thermal conductivity of 300 W/mK or less even if the conditions are optimized. Therefore, there is a need for development of the heat sink material having higher thermal conductivity. As such a material, a prior art proposes a metal-diamond composite having a high thermal conductivity and a coefficient of thermal expansion close to that of the semiconductor element, by a combination of the high thermal conductivity of diamond and the large coefficient of thermal expansion of the metal (Patent Document 2).
Further, a prior art discloses that forming of β-type SiC layers on surfaces of diamond particles suppresses generation of a metal carbide having a low coefficient of thermal expansion formed during production of a composite and improves wettability to a molten metal, thereby improving thermal conductivity of a diamond composite material obtained (Patent Document 3).
In the production of a metal-ceramics composite, a step of molding ceramics powder conventionally uses a powder filling method or a preform method. In the preform method, a porous molded body can be produced by sintering the ceramic particles or mixing them with an inorganic binder, molding them and then firing them, which is expected to improve dimensional accuracy. However, the power filling method has a problem that it cannot be applied to ceramic particles which cannot be sintered, such as graphite or diamond. Further, the preform method has a disadvantage that the heat conductivity of the heat sink is decreased because the inorganic binder is present in the resulting composite. To solve the problems, the applicant of the present application developed a process for producing an aluminum-diamond-based composite using the powder filling method, by placing diamond grains in the form of powder in a porous mold member, sandwiching them between dense mold release plates to which a mold releasing agent has been applied, to form a structured body, and carrying out molten metal forging with molten aluminum (Patent Document 4).